Salmonella and Salmonellosis (page 3)
(This chapter has 5 pages)
© Kenneth Todar, PhD
Isolation and Identification of Salmonella
A number of plating media have been devised for the isolation of Salmonella.
Some media are differential and nonselective, i.e., they contain
lactose
with a pH indicator, but do not contain any inhibitor for non
salmonellae
(e.g., bromocresol purple lactose agar). Other media are differential
and
slightly selective, i.e., in addition to lactose and a pH indicator,
they
contain an inhibitor for nonenterics (e.g., MacConkey agar and
eosin-methylene
blue agar).
The most commonly used media selective for Salmonella are SS
agar, bismuth sulfite agar, Hektoen enteric (HE) medium, brilliant
green
agar and xylose-lisine-deoxycholate (XLD) agar. All these media contain
both selective and differential ingredients and they are commercially
available.

Figure 3. Salmonella
sp. after 24 hours growth on XLD agar. Xylose Lysine (XL) agar is used
when trying to culture and isolate Gram-negative enteric bacilli. When
XL agar is supplemented with sodium thiosulfate, ferric ammonium
citrate,
and sodium deoxycholate, it is then termed XLD agar, and is then an
even
more selective medium than XL alone. The presence of any black colored
area indicates the deposition of hydrogen sulfide, (H2S)
under
alkaline conditions. (CDC)
Media used for Salmonella identification are those used for
identification
of all Enterobacteriaceae. Most Salmonella strains are
motile
with peritrichous flagella, however, nonmotile variants may occur
occasionally.
Most strains grow on nutrient agar as smooth colonies, 2-4 mm in
diameter.
Most strains are prototrophs, not requiring any growth factors.
However,
auxotrophic strains do occur, especially in host-adapted serovars such
as Typhi and Paratyphi A.

Figure 4. Colonial growth Salmonella
choleraesuis subsp. arizonae bacteria grown on a blood agar culture
plate. Also known as Salmonella Arizonae, it is a zoonotic
bacterium
that can infect humans, birds, reptiles, and other animals. (CDC)
Table 1. Characteristics
shared
by most Salmonella strains belonging to subspecies I
Motile, Gram-negative bacteria
Lactose negative; acid and gas from glucose,
mannitol, maltose, and sorbitol; no Acid from adonitol, sucrose,
salicin,
lactose
ONPG test negative (lactose negative)
Indole test negative
Methyl red test positive
Voges-Proskauer test negative
Citrate positive (growth on Simmon's citrate
agar)
Lysine decarboxylase positive
Urease negative
Ornithine decarboxylase positive
H2S produced from thiosulfate
Do not grow with KCN
Phenylalanine and tryptophan deaminase negative
Gelatin hydrolysis negative

Figure 5. Colonial growth
pattern
displayed by Salmonella Typhimurium cultured on a Hektoen
enteric
(HE) agar. S. Typhimurium colonies grown on HE agar are
blue-green
in color indcating that the bacterium does not ferment lactose However
it does produce hydrogen sulfide, (H2S), as indicated by
black
deposits in the centers of the colonies. (CDC). HE agar is the medium designed
for the isolation and recovery of fecal bacteria belonging to the
family,
Enterbacteriaceae. S.Typhimurium causes 25% of the 1.4 million
salmonellosis
infections a year in the United States. Most persons infected with
Salmonella
sp. develop diarrhea, fever, and abdominal cramps 12 - 72 hours after
infection.
The illness usually lasts 4 - 7 days, and most people recover without
treatment.
However, in some cases, the diarrhea may be so severe that the patient
needs to be hospitalized.
Genetics of Salmonella
The genetic map of the Salmonella Typhimurium strain LT2 is
not
very different from that of Escherichia coli K-12. The F
plasmid
can be transferred to Typhimurium, and an Hfr strain of Typhimurium may
subsequently be selected. Conjugative chromosomal transfer may occur
from
Typhimurium Hfr to E. coli or from E. coli Hfr to
Typhimurium.
Chromosomal genes responsible for O, Vi, and H antigens can be
transferred
from Salmonella to Escherichia.
Also, Salmonella may harbor temperate phages and
plasmids.
Plasmids in Salmonella may code for antibiotic resistance
(resistance
plasmids are frequent due to the selective pressure of extensive
antibiotic
therapy), bacteriocins, metabolic characteristics such as lactose or
sucrose
fermentation, or antigenic changes of O antigen.
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